1. Field of the Invention
This invention relates to a sanitary pipe gasket for use in a piping system associated with the manufacture of medicines, foods, etc., and more particularly to a sanitary-pipe gasket which is integrally formed of porous polytetrafluoroethylene.
2. Description of Related Art
As used herein, "sanitary" has reference to a barrier to contamination of medicines, foods, etc., by substances which adversely affect the living body, for example, foreign matter such as dust, impurities, bacteria, microorganisms, etc. Particularly, in order to provide a sanitary-pipe gasket which satisfies conditions for maintaining purity of contents, the International Organization for Standardization (ISO) provides that a structure having projecting portions 3 and 3' as shown in FIGS. 1 and 2 should be adopted to fix a sanitary-pipe gasket at a proper position so that no trap of liquid or the like occurs inside of the pipe.
As compared to a general use gasket, the material for a sanitary-pipe gasket to be used in manufacturing medicines, foods, etc. must be more carefully selected. This requirement is made to prevent contamination of products by components of the gasket material eluting into the fluid transported through the piping. Accordingly, most sanitary-pipe gaskets have conventionally been formed of silicone rubber which has excellent heat resistance and chemical resistance.
In the manufacture of medicine, etc., product management is strict, and in order to satisfy requirements, the manufacturing apparatus is cleaned at a higher frequency in comparison with apparatus used in other applications. For example, for sanitary apparatus, particularly that used in manufacturing medicines, an alkali cleaning treatment or a cleaning treatment using alkali and saturated vapor are frequently conducted to remove chemicals, microorganisms and pyrogens which would be injurious to a living body.
Silicone rubber which has been conventionally used for gaskets in sanitary piping has excellent acid resistance, however, but not alkali resistance, and, accordingly, there is the possibility that alkali components will remain on the surface of the gasket after the alkali cleaning treatment, thus causing various adverse post-treatment affects. Even when the amount of residual alkali component is very low, there is the disadvantage that the gasket may become fixed to the metal surface of a flange when the device is subjected to a steam disinfection treatment.
A sealing element of polytetrafluoroethylene (hereinafter referred to as "PTFE") resin, which has excellent heat resistance and mechanical resistance and lacks the above disadvantage of the silicone rubber gasket, has been proposed in the form of packing, a gasket, an O-ring and the like, which types of element are applicable, not only to a sanitary apparatus, but also to various other types of apparatus. Such a sealing element of PTFE resin is disclosed, for example, in Japanese Laid-Open Patent Application No. 63-242610 and No. 62-21819. Further, a sealing member of stretched porous PTFE has also been disclosed in Japanese Utility Model Publication No. 2-45594.
However, there has not yet been disclosed a gasket for a sanitary pipe (a pipe for sanitary apparatus) used in the production of medicine products, food products, etc., for which strict checking for the presence of eluted components as impurities in the products is required and repetitive cleaning under severe conditions is indispensable.
In attempting to develop a gasket suitable for a sanitary pipe, the inventors of this application tried various kinds of materials, and through these attempts, the inventors produced a sanitary-pipe gasket using PTFE resin, which has excellent alkali resistance, as a substitute for the conventional silicone rubber. However, since PTFE resin has higher rigidity than silicone resin, gas leakage through the gasket is liable to occur through gasket. As a consequence, the flanges are required to be tightened under higher pressure. In addition, flanges, once closely tightened against the gasket, are difficult to closely tighten again, and thus the prevention of gas leakage becomes extremely difficult. This is because of low elastic recovery of the PTFE gasket.